This will list the Windows Boot Manager entries:
C:\Windows\system32>bcdedit
Windows Boot Manager
--------------------
identifier {bootmgr}
device partition=\Device\HarddiskVolume2
description Windows Boot Manager
locale en-US
inherit {globalsettings}
default {current}
resumeobject {1476af5e-e5bc-11de-b180-0024543ae029}
displayorder {current}
{1476af63-e5bc-11de-b180-0024543ae029}
toolsdisplayorder {memdiag}
timeout 10
Windows Boot Loader
-------------------
identifier {current}
device partition=C:
path \windows\system32\winload.exe
description Windows 7
locale en-US
inherit {bootloadersettings}
recoverysequence {1476af60-e5bc-11de-b180-0024543ae029}
recoveryenabled Yes
osdevice partition=C:
systemroot \windows
resumeobject {1476af5e-e5bc-11de-b180-0024543ae029}
nx OptIn
Real-mode Boot Sector
---------------------
identifier {1476af63-e5bc-11de-b180-0024543ae029}
device partition=C:
path \ubuntu\winboot\wubildr.mbr
description Ubuntu
C:\windows\system32>
Note the Real-mode Boot Sector entry that has a description of Ubuntu. Copy and paste the identifier and then delete it as follows (in my case):
bcdedit /delete {1476af63-e5bc-11de-b180-0024543ae029}
Bookmarks